alt Hacker NewsThe term I've seen a lot is https://en.wikipedia.org/wiki/Unit_in_the_last_place
So I'd probably rewrite that code to first find the ulp of the larger of the abs of a and b and then assert that their difference is less than or equal to that.
Edit: Or maybe the smaller of the abs of the two, I haven't totally thought through the consequences. It might not matter, because the ulps will only differ when the numbers are significantly apart and then it doesn't matter which one you pick. Perhaps you can just always pick the first number and get its ULP.
This is what was done to a raytracer I used. People kept making large-scale scenes with intricate details, think detailed ring placed on table in a room with a huge field in view through the window. For a while one could override the fixed epsilon based on scene scale, but for such high dynamic range scenes a fixed epsilon just didn't cut it.
IIRC it would compute the "dynamic" epsilon value essentially by adding one to the mantissa (treated as an integer) to get the next possible float. Then subtract from that the initial value to get the dynamic epsilon value.
Definitely use library functions if you got 'em though.